1. Field of the Invention
The present invention relates to a method for stabilizing vinyl aromatic monomers using selected polymerization inhibitors and polymers prepared therewith. The present invention particularly relates to a method for stabilizing styrene monomer and polystyrene polymers prepared therewith.
2. Background of the Art
It is well known that vinyl aromatic compounds such as monomeric styrene, alpha-methyl styrene, and the like, polymerize readily and that the rate of such a polymerization increases with increasing temperature. Modern production methods for these and other vinyl aromatic compounds include separation and purification processes. Such separation and purification is often accomplished by distillation.
Various types of polymerization inhibitors have been employed to prevent polymerization during production and storage of vinyl aromatic compounds. For example, U.S. Pat. No. 4,466,904 to Watson, et al., discloses that inhibitors useful for inhibiting the polymerization of vinyl aromatic compounds include 4-tert-butylcatechol(TBC), phenothiazine, and 2,6-dinitro-p-cresol. TBC in particular is a preferred polymerization inhibitor for storage applications, having a good efficiency in preventing premature polymerization and no nitrogen and/or halides that can require special care during waste disposal. Such use of polymerization inhibitors to prevent polymerization of vinyl aromatic monomers is often referred to as stabilization and a monomer having an effective amount of a polymerization inhibitor present is referred to as being stabilized.
Some polymerization inhibitors work well in the absence of oxygen. 2,6-dinitro-p-cresolworks well as a polymerization inhibitor in a vinyl aromatic monomer in an oxygen free process. Others do not. Another class of chemical compounds that is useful as polymerization inhibitors in the production of vinyl aromatic monomers is the phenylenediamines. Certain polymerization inhibitors, such as phenylenediamine and TBC, require the presence of oxygen to inhibit polymerization of vinyl aromatic monomers.
A more recent class of inhibitors, described as 7-substututed quinone methides, has been disclosed in U.S. Pat. No. 5,750,765 to Nesvadba, et al., and U.S. Pat. No. 6,024,894 to Arhancet, et al. In Nesvadba, these polymerization inhibitors are disclosed to be more active than earlier described methides. In Arhancet, the 7-substututed quinone methides are disclosed as being particularly useful when used in combination with N,N-bis(hydroxypropyl)hydroxylamine.
The use of such polymerization inhibitors in general, and TBC in particular, is not trouble free. For example, for many applications, TBC must be removed prior to polymerization of vinyl aromatic monomers. One method of removing TBC from vinyl aromatic monomers is filtration through alumina. Another method of removing TBC from vinyl aromatic monomers is performed by washing the vinyl aromatic monomers with aqueous sodium or potassium hydroxide.